A tunable laser has a wavelength of operation of the laser light (optical signal) that may be altered during operation. A tunable laser typically requires a tunable filter that selects one of the possible longitudinal modes (mode selection filter) of an optical cavity of the tunable laser. In bulk optic designs, this mode selection filter is built using a reflection grating in first order (e.g., Littman-Metcalf or Littrow geometry). The mode selection filter serves basically two purposes. First, it first allows longitudinal single mode operation, as it increases losses for all other modes to an extent, to yield a good side mode suppression ratio (SMSR). Second, the mode selection filter filters out unwanted spontaneous source emission of a gain medium in the optical cavity when used such that the light coupled out of the optical cavity passes through the mode selection filter at least once.
For a continuously tunable laser, the oscillating wavelength of light in the optical cavity and the filter wavelength of the tunable filter need to match always. In bulk optical external cavity lasers using Littman-Metcalf or Littrow geometry, for example, such matching may be achieved by proper mechanical design and alignment of the optical cavity elements. However, it is difficult to maintain this condition over time, due in part to variations of ambient temperature and wavelength. Hence, additional alignment actors are typically incorporated that are either driven by calibration values or by sensors that measure the deviation of the tunable filter wavelength versus the lasing wavelength. The additional alignment may include passive alignment or active alignment taking advantage of a sensor. The tunable filter can be used independently from the tunable laser as “clean-up filter” to remove unwanted spontaneous source emissions from the laser signal.
Conventional tunable lasers generally use a bulk optical filter or a filter that is not operated in closed loop (thus, operated in open loop mode). Using bulk optical filters leads to large and expensive optics with a number of design disadvantages, such as lower speed, sensitivity to vibration and environmental change, large footprint, and high cost due to expensive parts and extensive labor to obtain alignment. Operating the bulk optical filter in open loop mode requires a calibration of the filter wavelength, which needs to be stable over time and in various environmental conditions. Depending on the design, these requirements may be impossible or at least very difficult to fulfill. Also, there is the risk that the optical filter is not operated at the correct wavelength, leading to higher losses inside the optical cavity, which in turn leads to worsening SMSR, higher spontaneous source emission, and lower output power, or worse, the laser operation is blocked. For wideband tuning, a higher cavity loss also leads to a smaller tuning range since the gain of the gain medium is lower at the upper and lower wavelength ends of the gain profile that must compensate for the cavity losses.
What is needed, therefore, is a tunable optical filter that is efficiently and continuously tunable, such that resonance of the tunable optical filter is tuned such that the tunable optical filter is in resonance with an input optical signal to be filtered within the optical cavity.